minor #34990 Use ::class constants instead of __NAMESPACE__ when possible (fre5h)

This PR was merged into the 3.4 branch.

Discussion
----------

Use `::class` constants instead of `__NAMESPACE__` when possible

| Q             | A
| ------------- | ---
| Branch?       | 3.4
| Bug fix?      | no
| New feature?  | no
| Deprecations? | no
| Tickets       | Related to #34987
| License       | MIT
| Doc PR        | no

Form component has a lot of built-in form types. Some of them were implemented from the very beginning. In most of them there is a such method

```php
    /**
     * {@inheritdoc}
     */
    public function getParent()
    {
        return __NAMESPACE__.'\TextType';
    }
```

This `getParent()` method was refactored in Symfony 2.8. The upgrade instructions are given here https://github.com/symfony/symfony/blob/2.8/UPGRADE-2.8.md#form

I think the `__NAMESPACE__.'\TextType';` expression was used because Symfony 2.8 was using `"php": ">=5.3.9"`, and the constant `::class` was added only in PHP 5.5

Now this line can be refactored into
```php
    /**
     * {@inheritdoc}
     */
    public function getParent()
    {
        return TextType::class;
    }
```

For example new form types, that were added later, already using the `::class` constant.
https://github.com/symfony/symfony/blob/master/src/Symfony/Component/Form/Extension/Core/Type/ColorType.php#L23
https://github.com/symfony/symfony/blob/master/src/Symfony/Component/Form/Extension/Core/Type/TelType.php#L23

So, in this pull request I propose to refactor all old form types to use `::class` constant. It will give a benefit during the future refactoring, because IDE or static analysers will find all usages of parent class. Unlike the `__NAMESPACE__.'\TextType';` line, which doesn't show the real link to the class for IDE or static analysers, and it could complicate finding all usages of parent class.

Commits
-------

32bf50abca Use `::class` constants instead of `__NAMESPACE__` when possible

Author: nicolas-grekas

Validator/Constraints/UserPasswordValidator.php

@@ -36,7 +36,7 @@ public function __construct(TokenStorageInterface $tokenStorage, EncoderFactoryI
     public function validate($password, Constraint $constraint)
     {
         if (!$constraint instanceof UserPassword) {
-            throw new UnexpectedTypeException($constraint, __NAMESPACE__.'\UserPassword');
+            throw new UnexpectedTypeException($constraint, UserPassword::class);
         }
 
         if (null === $password || '' === $password) {